1. Field of the Invention
This invention relates to a process for producing a radiation image storage panel for use in a radiation image recording and reproducing techniques, in which stimulable phosphors are utilized.
2. Description of the Related Art
In lieu of conventional radiography, radiation image recording and reproducing techniques utilizing stimulable phosphors have heretofore been used in practice. The radiation image recording and reproducing techniques are described in, for example, U.S. Pat. No. 4,239,968. The radiation image recording and reproducing techniques utilize a radiation image storage panel (referred to also as the stimulable phosphor sheet) provided with a stimulable phosphor. With the radiation image recording and reproducing techniques, the stimulable phosphor of the radiation image storage panel is caused to absorb radiation, which carries image information of an object or which has been radiated out from a sample, and thereafter the stimulable phosphor is exposed to an electromagnetic wave (stimulating rays), such as visible light or infrared rays, which causes the stimulable phosphor to produce the fluorescence (i.e., to emit light) in proportion to the amount of energy stored thereon during its exposure to the radiation. The produced fluorescence (the emitted light) is photoelectrically detected to obtain an electric signal. The electric signal is then processed, and the processed electric signal is utilized for reproducing a visible image of the object or the sample.
The radiation image recording and reproducing techniques have the advantages in that a radiation image containing a large amount of information can be obtained with a markedly lower dose of radiation than in the conventional radiography utilizing a radiation film and an intensifying screen. Therefore, the radiation image recording and reproducing techniques are efficient particularly for direct medical radiography, such as the X-ray image recording for medical diagnosis.
Basically, the radiation image storage panel utilized for the radiation image recording and reproducing techniques comprises a substrate and a stimulable phosphor layer overlaid on one surface of the substrate. Ordinarily, the stimulable phosphor layer comprises a binder and stimulable phosphor particles dispersed in the binder. The stimulable phosphor has the properties such that, when the stimulable phosphor is caused to absorb radiation, such as X-rays, and is then exposed to the stimulating rays, the stimulable phosphor emits light in proportion to the amount of energy stored thereon during its exposure to the radiation. Therefore, when the radiation image storage panel is exposed to the radiation, which carries image information of an object or which has been radiated out from a sample, the stimulable phosphor layer of the radiation image storage panel absorbs the radiation in proportion to the dose of radiation, and a radiation image of the object or the sample is stored as an image of energy from the radiation on the radiation image storage panel. The radiation image storage panel is then exposed to the stimulating rays, and the image having been stored on the radiation image storage panel can be detected as the light emitted by the radiation image storage panel. The emitted light is detected photoelectrically to obtain an image signal, the image signal is processed, and the thus obtained processed image signal can then be utilized for reproducing the radiation image of the object or the sample as a visible image.
As described above, the radiation image recording and reproducing techniques are the advantageous image forming techniques. However, as in the cases of an intensifying screen employed in the conventional radiography, it is desired that the radiation image storage panel utilized for the radiation image recording and reproducing techniques has a high sensitivity and can yield an image of good image quality (with respect to sharpness, graininess, and the like).
Basically, the sensitivity of the radiation image storage panel depends upon a total light emission intensity of the stimulable phosphor, which is contained in the radiation image storage panel. The total light emission intensity depends upon light emission luminance of the stimulable phosphor itself and the content of the stimulable phosphor in the stimulable phosphor layer. In cases where the content of the stimulable phosphor in the stimulable phosphor layer is high, a high absorptivity is capable of being obtained with respect to the radiation, such as the X-rays. Therefore, in cases where the content of the stimulable phosphor in the stimulable phosphor layer is high, a high sensitivity is capable of being obtained, and the image quality (in particular, with respect to the graininess) is capable of being enhanced. In cases where the content of the stimulable phosphor in the stimulable phosphor layer is kept at a predetermined value, if the stimulable phosphor particles are packed closely in the stimulable phosphor layer, the thickness of the stimulable phosphor layer can be set to be thin. Therefore, in such cases, spreading of the stimulating rays due to scattering in the stimulable phosphor layer is capable of being minimized, and an image having a high sharpness is capable of being obtained.
As radiation image storage panels provided with a stimulable phosphor layer, in which a stimulable phosphor is packed closely, and processes for producing the radiation image storage panels, the applicant proposed radiation image storage panels, in which a void content in the stimulable phosphor layer has been reduced by performing compression processing on the stimulable phosphor layer, and processes for producing the radiation image storage panels. The proposed radiation image storage panels and the proposed processes for producing the radiation image storage panels are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 59(1984)-126299 and 59(1984)-126300.
With the proposed radiation image storage panels, the stimulable phosphor layer is subjected to the compression processing, the density of the stimulable phosphor in the stimulable phosphor layer is thereby set at a higher value than in conventional radiation image storage panels, and nonuniformity in stimulable phosphor density in the stimulable phosphor layer is thereby reduced. As a result, an image having a high sharpness is capable of being obtained with the proposed radiation image storage panels.
However, subsequent research revealed that, with the compression processing alone, nonuniformity in stimulable phosphor density in the stimulable phosphor layer cannot always be eliminated perfectly. It has thus been found that an improvement should be made even further with respect to nonuniformity in light emission of the radiation image storage panel due to nonuniformity in stimulable phosphor density.
The inventors conducted extensive research with regard to techniques for preparing a coating composition for the formation of the stimulable phosphor layer and found that it is markedly efficient to employ a mixed solvent, which comprises at least two kinds of organic solvents having different boiling temperatures and different viscosities, as an organic solvent for use in preparation of the coating composition for the formation of the stimulable phosphor layer. The present invention is based on such findings.